Hydrogenation processes for the complete saturation of polynuclear aromatic compounds are well known in the art. For example, naphthalene can be hydrogenated to decalin and quinoline or isoquinoline can be hydrogenated to decahydroquinolines. However, as is well recognized in the art, the selective hydrogenation of only the more readily reducible sites in a feedstock characterized by the presence of different types of unsaturation is not readily accomplished. In general, a complex mixture of products exhibiting varying degrees of saturation is obtained. Catalytic hydrogenation systems possessing the capacity for effecting the selective hydrogenation of nitrogen heterocyclic rings in heterocyclic aromatic compositions are significant contributions to the art because practical routes to useful products can be developed and other processes such as hydrodenitrogenation (HDN) can be rendered more economical.
In a HDN process, the nitrogen in organic nitrogen compounds is converted to ammonia. This nitrogen must be removed by the process of hydrodenitrogenation (HDN) to prevent poisoning of refining catalysts and to avoid the sale of products which form gums and sediments or cause air pollution on burning. Heavy oil, shale oil and coal-derived liquids contain high levels of such nitrogenous compositions. If the feedstream to a HDN process contains homocyclic aromatics as well as heterocyclic aromatic compounds characterized by the presence of unsaturated nitrogencontaining rings, significant amounts of hydrogen can be consumed by the undesirable hydrogenation of the homocyclic aromatics. This undesired nonselective hydrogenation reaction increases the overall cost of the process because hydrogen is expensive. Current HDN methods consume more hydrogen than necessary to remove nitrogen as ammonia because in a typical hydrodenitrogenation process homocyclic aromatic rings are hydrogenated. Thus, HDN processes based on more selective hydrogenation catalysts need to be developed and would be highly preferred.
One value of the instant selective hydrogenation process resides in its potential use as the initial step of a hydrodenitrogenation operation. The application of the instant process as the front end of a HDN operation would result in hydrogenation of the unsaturated nitrogen-containing rings in the heterocyclic aromatic compositions without promoting hydrogenation of the homocyclic aromatics. This would result in lower process costs because less hydrogen would be consumed. The second step of the HDN process involving hydrogenolysis of C-N bonds to complete the removal of nitrogen as ammonia would be effected by other chemical means.